There is a phenomenon whereby electroluminescent emission is obtained by applying a high voltage to a compound semiconductor such as ZnS and ZnSe doped with a luminous center such as Mn. Recently, by the development of thin-film EL devices of a double insulating structure, luminance and life have been rapidly improved as described in SID 74 Digest of Technical Papers p. 84, 1974 and Journal of Electrochemical Society, 114, 1066 (1967), and such thin-film EL devices are employed for flat panel displays which are now commercially available.
The emission color of the EL devices is determined by the combination of a semiconductor host material constituting a phosphor layer and a luminous center. For example, the ZnS:Mn phosphor layer in which ZnS is a host material and Mn is a luminous center exhibits a yellow-orange electroluminescent emission (hereinafter referred to as "EL emission") and the ZnS:Tb phosphor layer exhibits a green EL emission. For the preparation of full color thin-film displays with EL devices, there are EL devices which emit the three primary colors, i.e., red, blue and green. High luminance red, blue or green emitting EL devices have been investigated. With regard to the blue color, it is known that blue EL emission can be obtained from a ZnS:Tm phosphor layer and a SrS:Ce phosphor layer as in Japanese Patent Publication (Kokoku) No. 46117/1988 and Hiroshi Kobayashi, THE JOURNAL OF THE INSTITUTE OF TELEVISION ENGINEERS OF JAPAN, 40, 991 (1986).
However, the luminance of these EL devices is not sufficient, and of these EL devices the luminance of the blue EL devices is particularly low. According to Japanese Patent Publication (Kokoku) No. 46117/1988, a luminance of about 100 fL (350 cd/m.sup.2) with 2.5 kHz drive frequency is obtained with an EL device having a SrS:Ce phosphor which has been prepared by electron beam evaporation and has been annealed at 600.degree. C. for 30 minutes in a hydrogen sulfide atmosphere. According to SID 86 Digest of Technical Papers, p. 29, 1986, a maximum luminance of 1600 cd/m.sup.2 with 5 kHz drive frequency is attained with the EL device having a SrS:Ce phosphor layer which has been prepared by the electron beam evaporation in a sulfur atmosphere, and this luminance value is the highest so far obtained. For practical purposes, however, this value is still very low and the conditions for preparing high luminance EL devices have been investigated. For example, phosphor layers having high crystallinity can be prepared by molecular beam epitaxy (MBE) or metal organic chemical vapor deposition (MOCVD), and by these methods a considerably high luminance is obtained with the yellow-orange emitting EL device having a ZnS:Mn phosphor layer. But as for the blue emitting EL device having a SrS:Ce phosphor layer, a high luminance has not been obtained.
In the present invention, it has been found that when the phosphor layer of high luminance EL devices having a phosphor layer comprising SrS as the host material is annealed at a temperature of at least 650.degree. C. for at least one hour in an atmosphere of a sulfur-containing gas, the phosphor layer exhibits a characteristic peak in the neighborhood of a wavelength 360 nm in the excitation spectrum and the EL device having such an annealed phosphor layer shows a high luminance.